Conventional, thermal, flow measuring devices use, most often, two, as much as possible, equally embodied, temperature sensors. For industrial application, the two temperature sensors are usually installed in a measuring tube, in which the flow of a measured medium is measured. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. Provided as heating unit is either an additional resistance heating element, or the temperature sensor is itself a resistance element, e.g. an RTD (Resistance Temperature Device) sensor, which is heated by conversion of electrical power, e.g. by a corresponding variation of the measuring, electrical current. The second temperature sensor is a so-called passive temperature sensor: It measures the temperature of the medium.
Usually, in a thermal, flow measuring device, the heatable temperature sensor is so heated, that a fixed temperature difference is set between the two temperature sensors. Alternatively, it is also known to supply, via an open, or closed, loop control unit, a constant heating power.
If there is no flow in the measuring tube, then an amount of heat constant with time is required for maintaining the predetermined temperature difference. If, in contrast, the medium to be measured is moving, then the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, heat is transported away from the heated temperature sensor by the flowing medium. In order, thus, to maintain the fixed temperature difference between the two temperature sensors in the face of a flowing medium, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow, i.e. the mass flow of the medium through the pipeline.
If, in contrast, a constant heating power is fed, then the temperature difference between the two temperature sensors lessens as a result of the flow of the medium. The particular temperature difference is then a measure for the mass flow of the medium through the pipeline, or through the measuring tube.
There is, thus, a functional relationship between the heating energy needed for heating the temperature sensor and the mass flow through a pipeline, or through a measuring tube. This dependence of the heat transfer coefficient on the mass flow of the medium through the measuring tube, or through the pipeline, is utilized in thermal, flow measuring devices for determining the mass flow. Devices, which operate according to this principle, are available from the assignee under the mark ‘t-mass’.
In order to achieve a desired high accuracy of measurement, thermal, flow measuring devices are first calibrated against a highly accurately measuring master, for example, against a Coriolis, flow measuring device. The known calibration method is based on the assumption that the heat exchange between temperature sensor and medium occurs exclusively via forced convection. This assumption loses its validity in the region of smaller flow velocities of the medium. Investigations have shown, that, exactly in the region of smaller flow velocities, heat exchange as a result of free, or natural, convection plays a large role. Therefore, correction based on purely forced convection is associated with relatively large measurement errors.
From EP 0 624 242 B1, already a thermal, flow measuring device is presented, which takes into consideration the heat transfer between fluid and temperature sensor as a result of free convection. In the region of smaller flow velocities, thus, especially, in the case of flow velocities smaller than 1 m/sec, there occurs, besides the forced convection, additionally heat transfer based on free convection. If this heat transfer as a result of free convection is ignored, then relatively large measurement errors occur in the case of small flow velocities.